The present invention relates to novel fluid loss control agents for use in drilling, completion and stimulation fluids. More particularly the present invention relates to the use of a slurry of particulate crosslinked polymer gels to form a filter cake to reduce treatment fluid loss to permeable formations.
During the drilling of an oilwell, a usually aqueous fluid is injected into the well through the drill pipe and recirculated to the surface in the annular area between the wellbore wall and the drill string. The functions of the drilling fluid include: lubrication of the drill bit, transportation of cuttings to the surface, overbalancing formation pressure to prevent an influx of oil, gas or water into the well, maintenance of hole stability until casings can be set, suspension of solids when the fluid is not being circulated and minimizing fluid loss into and possible associated damage/instability to the formation through which drilling is taking place.
Proper overbalancing of formation pressure is obtained by establishing fluid density at the desired level usually via the addition of barite. Transportation of cuttings and their suspension when the fluid is not circulating is related to the fluid viscosity and thixotropy which depend on solids content and/or use of a polymer. Fluid loss control is obtained also by the use of clays and/or added polymers.
Fluid properties are constantly monitored during the drilling operations and tailored to accommodate the nature of the formation stratum being encountered at the time. When drilling reaches the producing formation special concern is exercised. Preferentially, low solids content fluids are used to minimize possible productivity loss by solids plugging. Proper fluid density for overbalancing formation pressure may be obtained by using high salt concentration aqueous brines while viscosity and fluid loss control generally are attempted by polymer addition.
When high permeability and/or poorly consolidated formations are penetrated as the zone of interest, a technique referred to as "under-reaming," often is employed in the drilling operations. In this process, the wellbore is drilled through the hydrocarbon bearing zone using conventional techniques and drilling muds. A casing generally is set in the wellbore to a point just above the hydrocarbon bearing zone. The hydrocarbon bearing zone then is redrilled using an expandable bit that increases the diameter of the hole. The purpose of the under-reaming is to remove damage from the permeable formation introduced during the initial drilling process by particles of the drilling mud and to increase the exposed surface area of the wellbore. Typically, under-reaming is effected utilizing special "clean" drilling fluids to minimize further formation damage. The high permeability of many hydrocarbon zones allows large quantities of the clean drilling fluid to be lost to the formation. Typical fluids utilized in under-reaming comprise aqueous brines which are viscosified with a polysaccharide polymer to aid in the removal of the drill cuttings. To control fluid loss, solids such as ground salt crystals, calcium carbonate or the like may be added to the fluid to form a filter cake on the face of the formation. This filter cake must be removed prior to or after the zone is gravel packed if hydrocarbon production is to be achieved. The salt crystals can be removed by circulating unsaturated salt brine to dissolve the particles. If this occurs before gravel packing, the circulating fluid often causes sloughing of the formation into the wellbore and yet further loss of fluids to the formation. If removal is attempted after the gravel pack, the gravel packing material often traps the particles against the formation and makes removal much more difficult. Other particulates, such as the carbonates can be removed with circulation of acid, however, the same problems may arise.
Completion fluids are those fluids used after drilling is complete and during the steps of completing the well. Completion can include cementing the casing in the well, perforating the well, setting the tubing, pump and the like. Completion fluids are generally used to control the well pressure, provide fluid loss control, prevent the collapse of tubing from overpressure and to reduce or prevent corrosion of the casing when left in a wellbore as a packer fluid above the production packer. In such an instance the fluid is placed in the annulus between the casing and production tubing string above the production packer to seal the annulus against the influx of formation fluids. Because a packer fluid may remain in a wellbore for many years before any remedial work is needed, such a fluid generally should be temperature stable, non-settling, non-corrosive and non-damaging to the formation.
Stimulation of hydrocarbon bearing formations typically is effected to increase the exposed area of a subterranean formation for flow of a hydrocarbon to a wellbore. Commonly, fracturing operations are performed to produce conductive channels from the wellbore into the formation. The channels may have a particulate proppant introduced therein to assist in maintaining the channels open within the formation. While creating fractures in low permeability formation is often considered a routine technique, the formation of fractures in higher permeability formations (500 millidarcy and greater) presents several problems. Effective fluid loss control is essential to creation of a fracture of any length in a high permeability formation. Typical fluid loss control additives for stimulation fluids comprise the ground salt, ground calcium carbonate and the like previously discussed in regard to clean drilling fluids. These materials are difficult to remove from the created fractures, particularly after the fracture is propped open by the introduction of a propping agent. The presence of unremoved fluid loss additives can result in a significant reduction in the production flow capacity of the created fracture. If an attempt is made to increase the stimulation fluid viscosity to a level whereby fluid loss can be controlled without significant quantities of particulate fluid loss additives, the friction pressures resulting from the higher viscosity fluid limit the pumping rate and diminish the ability to produce a desired fracture length.
The present invention has been developed with a view to providing improved drilling fluids, completion fluids and stimulation fluids that create and effectively maintain a buffer zone in a wellbore.